An outbreak of the mumps has continued to grow in Washington state, with at least 367 people diagnosed or suspected of having the illness, according to the Washington State Department of Health.

The vast majority of those infected have been school-age children, according to Paul Throne, the manager of the state's immunization program. Of the school-age children infected in this outbreak, 87 percent were up to date on their mumps vaccinations, Throne said today, but lower vaccination rates among school-age children in general may be contributing to the growth of the outbreak.

Despite the large number of mumps cases, health officials believe the vaccine is providing protection against more serious mumps complications, Throne told ABC News.

"We do think it's still protecting people who get sick. We have not seen the serious side effects that you might expect in an outbreak," he said.

The state health department has been grappling with the outbreak since October and has asked people in multiple counties to get vaccinated against the disease or stay home from school in the hopes of stopping the spread of the virus.

Mumps spreads through small droplets of water in the air — similar to the seasonal flu. The virus can be spread via sharing drinks or food or being in close contact with an infected person. Mumps can cause swelling of the salivary glands, resulting in enlarged cheeks and jaws. Additionally, it can cause fever, headache and tiredness. In rare cases, it can lead to meningitis, swelling of the brain and deafness. It can also cause death.

The Washington State Department of Health is working with the U.S. Centers for Disease Control and Prevention to determine if the current strain of the virus has changed at the DNA level, which could mean the vaccine is less effective, Throne said.

"We don't know if the mumps virus that we're seeing has shifted or drifted from strain in the vaccine. That's something that we're looking at," he said. "It's possible that the exact genotype is not a perfect match."

However, health officials believe the vaccine is broad enough to provide some protection against the virus, Throne said.

Two full doses of the mumps vaccine — usually given as part of the measles, mumps, rubella (MMR) vaccination — provide approximately 88 percent protection, and a single dose can provide 78 percent protection against mumps, according to the CDC.

However, diminished MMR vaccination rates among schoolchildren over age 5 in Washington state may be a contributing factor in the outbreak, Throne said.

Since the mumps vaccine is 88 percent effective even when properly administered, it can mean people can be infected with the virus even if vaccinated. If 12 out of 100 vaccinated people can get the virus, when others in the community decide not to get vaccinated, then it can dramatically increase the chance that the virus can spread even among vaccinated people, he noted.

"We have a lot of children in Washington whose [parents] have chosen to exempt children from vaccine" requirements, Throne said. "These are kids who are vulnerable to be exposed and to spread disease before they know they are sick."

The nature of the virus has made it difficult to stop, he said. Sick people can be contagious seven days before and eight days after they show symptoms. As a result, an infected person can unwittingly infect many others.

Additionally, the health department has had to educate medical providers about early signs of mumps, since many physicians had never seen the disease in a patient before, Throne said.

"We hadn't had mumps in Washington in a long time," he said, noting that the outbreak is expected to keep growing, despite the health department's best efforts.

"It's been a continuous upwards track," Throne said of new cases. "Until we reach a point where no more vulnerable people are exposed, it may continue to grow."